In the past, there has been proposed a lighting fixture that includes a light source constituted by at least one light emitting diode (LED) and sends a signal by modulating an intensity of illuminating light. Such a lighting fixture is disclosed in document 1 (JP 2012-069505 A), for example. This type of illuminating light communication device modulates the illuminating light itself, and thereby sends a signal. Hence, a special device such as an infrared communication device is unnecessary. Further, use of light emitting diodes as lighting sources for illumination can save electric power. For these reasons, it is being planned to use the aforementioned illuminating light communication device in a ubiquitous information system installed in premises such as an underground mall.
The illuminating light communication device disclosed in document 1 includes a constant current source, a smoothing capacitor, a load circuit, a load variable element, a signal generation circuit, and a switching element. The smoothing capacitor smooths output from the constant current source. The load circuit includes a plurality of light emitting diodes and is to receive the output from the constant current source. The load variable element is constituted by a resistor connected in parallel with one or some of the light emitting diodes. When the load variable element is connected to the load circuit, load characteristics of the load circuit are changed partially. The signal generation circuit is to generate a binary communication signal. The switching element is a switching device connected in series with the resistor serving as the load variable element. To select whether the load variable element is connected to the load circuit, the switching element is turned on and off depending on the communication signal. Accordingly, the load characteristics of the load circuit is varied in accordance with the communication signal, and therefore the load current flowing through the light emitting diode is modulated with a waveform of the communication signal.
However, as for a lighting fixture including the illuminating light communication device of the background art, the following undesired situation may occur. Namely, in a case where a plurality of lighting fixtures are installed so as to illuminate a large space such as an underground mall for example, when rays of illuminating light from the plurality of lighting fixtures overlap each other, such rays of illuminating light interfere with each other, and therefore a receiving terminal such as a mobile phone fails to successfully receive the communication signal. This is because that the illuminating light communication device of the background art sends the communication signals consecutively.
To avoid such an undesired situation, for example, there has been proposed a system in which the lighting fixtures are connected to each other by signal lines, and synchronization signals are given to the lighting fixtures to allow the lighting fixtures to transmit the communication signals at different timings to avoid interference. However, according to this proposal, it is troublesome to construct the system when the lighting fixtures are situated in places where connection of the signal lines is difficult. Moreover, the system is not available for the lighting fixtures which are devoid of components to be connected to the signal lines. Additionally, according to this system, it is necessary to add the signal lines to provide the synchronization signals to the illuminating light communication devices, and therefore the installation of the system is troublesome and also it is difficult to install the system especially in large-scale premises.